What is it about the letter that fascinates people and triggers different meanings in their brains?

Universal variable

Any school kid that learned algebra has been introduced to X as the consummate symbol for a variable. This gives the letter an ambiguity that allows for a variety of meanings and values.

Mystery or unknown

On a 50’s TV show called “What’s My Line” a celebrity panel sometimes had to guess the profession of guests that had famous names but not well-known faces. This guests signed is as Mr. or Ms. X.

Generic brand or unnamed competitor

Brand X typically refers to a generic brand. In a comparison test, brand X also refers to an unnamed competitor. Some say that Microsoft created the name Xbox 360 to promote the idea that it is a generic console that could play a lot of game software. Others say they derived the name from looking at the game controller of the Sony Playstation, where the X, the square, and the circle translate to Xbox 360.

Prevention or elimination

As in the product, Gas-X, the X refers to elimination or prevention of something. In this context X refers to erasing or crossing out of gas.

Superheroes

In the Marvel comics or movie by the same name, X-men are a collection of superheroes that started out as mutants that had the x-gene, and as a result had extra powers.

Experimental or edgy

The X-15 program was an experimental program that led to the American Space Program. Top pilots were enlisted to fly the experimental rocket-powered space planes to break speed records and measure the effect of high-altitude, high-speed flight on these planes and the pilots that flew them.

Former relationship

It can also stand for a former relationship, such as an ex-wife, ex-husband, ex-CEO, ex-friend, or ex-band member. To make the prefix shorter, some substitute the letter X, and in conversation, they typically say my X when referring to ex-spouse, boyfriend, or girlfriend

Label of a Generation

In recent times, Generation X has been used to describe the generation born between 1964 and 1981. It is the generation that officially marks the end of the Baby Boom generation. In contrast to Baby Boomers, some have referred to Generation X as the Baby Bust generation because it marked a declining birth rate that began in 1964.

Marking the spot

The letter X can also mark a spot that serves as a target for skydivers to land, a place where actors should be on a stage, or a place where a document should be signed and dated.

Making a selection

When people vote for a candidate or make a selection by checking a pre-printed box, they often indicate their selection by putting an X inside the box.

Placeholder

Similar to marking a spot or target, it can also be a placeholder to be filled in later. It can also mark the place for someone to drill a hole or place a nail.

Universal signature

For those that cannot read or write, X is often used as a universal mark to indicate approval, identity, or agreement on a document or contract.

Shortened version of Ex

The letter “X” is often a shorthand version of a longer prefix, such as ex.

Least number of words beginning with it

Because the least number of words in the English alphabet begin with X, this adds to its uniqueness and cachet. It also adds to X’s power. Less becomes more because it carries more weight.

Versatility and power for brand marketers

The versatility of “X” makes it interesting, mysterious, fascinating, and universal as a symbol that can be used very effectively in branding of companies, products, and ideas. This versatility contrasts with its uniqueness to give X a lot of power that brand marketers can use to their advantage.